1. Field of the Invention
The present invention relates to improvements in power converters for battery charging systems. More particularly, the invention relates to improvements particularly suited for lightweight high-temperature high-stress environments for electronics with high reliability requirements.
2. Description of the Known Art
As will be appreciated by those skilled in the art, batteries and chargers for those batteries are known in various forms. Lithium-ion batteries require cell balancing to achieve proper charging and discharging over the life of the battery pack. In a typical application, lithium-ion battery cells are connected in series to achieve the desired nominal voltage from the resulting battery. One would think that a simple charger of the appropriate voltage could charge the series string of batteries. However, variations in individual cells' maximum capacity prevent all the cells in a series string from being properly charged without damaging the weaker cells. In order to avoid damaging overcharging events, a cell balancing and bypass circuit is needed. Cell balancing circuits ensure that all Li-ion cells are optimally charged to their maximum level by monitoring each cell's state of charge and individually charging them to their optimal level while keeping already fully charged cells from overcharging. Currently, there are several hardware implementations to achieve cell balancing such as current bypass circuits and charge redistribution circuits. Current bypass or charge shunt circuits suffer from high inefficiency associated with the dissipative elements used to accomplish the current bypass.
Patents disclosing information relevant to batteries and chargers include: U.S. Pat. No. 8,421,400, issued to Khanna on Apr. 16, 2013, entitled Solar-powered battery charger and related system and method; U.S. Pat. No. 8,358,108, issued to Seman, Jr., et al. one Jan. 22, 2013 entitled System and method for re-initiating charge cycle for battery pack left in a charger; U.S. Pat. No. 8,098,048, issued to Hoff on Jan. 17, 2012 entitled Battery charger with integrated cell balancing; U.S. Pat. No. 8,044,640, issued to Cruise, et al. on Oct. 25, 2011 entitled Method for stepping current output by a battery charger; U.S. Pat. No. 7,880,435, issued to Ibrahim on Feb. 1, 2011 entitled Multi-battery charger with individual battery bypass control; U.S. Pat. No. 7,602,146, issued to Carrier, et al. on Oct. 13, 2009 entitled Protection methods, protection circuits and protection devices for secondary batteries, a power tool, charger and battery pack adapted to provide protection against fault conditions in the battery pack; U.S. Pat. No. 7,508,171, issued to Carrier, et al. on Mar. 24, 2009 entitled Protection methods, protection circuits and protective devices for secondary batteries, a power tool, charger and battery pack adapted to provide protection against fault conditions in the battery pack; U.S. Pat. No. 6,121,751, issued to Merritt on Sep. 19, 2000 entitled Battery charger for charging a stack of multiple lithium ion battery cells; and U.S. Pat. No. 5,969,502, issued to Beard on Oct. 19, 1999 entitled Battery charger capable of independently charging electromagnetic cells. Each of these patents is hereby expressly incorporated by reference in their entirety.
From these prior references it may be seen that these prior art patents are very limited in their teaching and utilization, and an improved power converter and charging system is needed to overcome these limitations.